Seeking the lowest possible variation of rate for a mechanical timepiece movement is known. However, it is very difficult to achieve this particularly because of the sensitivity of the sprung balance assembly to variations in temperature and magnetic fields.
EP Patent No. 1 039 352 discloses an alloy balance spring made of a particular alloy, whose external surface has an oxide coating. The document discloses a balance spring, which is thermocompensated, i.e., its thermoelastic coefficient, also called the Young's modulus thermal coefficient, remains approximately close to zero, and whose sensitivity to magnetic fields is very low. However, the spring is very difficult to implement, which leads to a very high reject rate and cost price.